The present invention relates to a pneumatic tire, more particularly to a tread structure being capable of improving wet performance and noise performance.
In order to improve wet performance of a tire, the volume of circumferential grooves and lateral grooves in the tread portion may be increased, but so called air-tube resonance is liable to occur and so called pitch noise increases. The air-tube resonance is a resonance of air in a circumferential groove in the ground contacting patch of the tire. The pitch noise is a noise generated by periodical contact of lateral grooves with a road surface.
Conventionally, to reduce such pitch noise, circumferential pitches of the lateral grooves are varied irregularly around the circumference of the tire. However, to maintain the required resistance to uneven wear, steering stability and the like, variation can not be so large. Thus, the pitch noise reduction by pitch variation has its limit. As to the air-tube resonance noise, it can be reduced by decreasing the main circumferential groove width or employing a zigzag circumferential groove, but in either case the wet performance deteriorates. Therefore, based on a compromise between noise performance and wet performance, the width and volume of the circumferential grooves are determined.
It is therefore, an object of the present invention to provide a pneumatic tire in which both the wet performance and noise performance are improved.
According to the present invention, a pneumatic tire comprises
a tread portion provided between the tire equator and each tread edge with a circumferentially continuous main circumferential groove and circumferentially spaced outer lateral grooves,
the outer lateral grooves including wide lateral grooves and narrow lateral grooves arranged alternately in the tire circumferential direction, each said outer lateral groove extending axially outwardly from the main circumferential groove while inclining at a certain angle with respect to the circumferential direction of the tire,
the main circumferential groove having an axially inner groove-wall and an axially outer groove-wall,
said axially inner groove-wall extending substantially straight in the circumferential direction,
said axially outer groove-wall comprising circumferential portions and inclined portions each between the wide lateral grooves and narrow lateral grooves,
said circumferential portions extending substantially straight in the circumferential direction so that the width of the main circumferential groove is substantially constant, and
said inclined portions inclined so that the width of the main circumferential groove increases from the narrow lateral grooves to the wide lateral grooves, whereby
the main circumferential groove has alternate constant width portions and widening portions.
Therefore, pitch noise is dispersed into a wider frequency range and improved owing to the alternate wide and narrow lateral grooves. The alternate constant width portions and widening portions prevent the occurrence of air resonance in the main circumferential groove and the air resonance noise can be improved. Further, in the widening portions, as the width of the main circumferential groove increases from the narrow lateral grooves towards the wide lateral grooves, the drainage from the main circumferential groove to the wide lateral grooves is increased, and the wet performance, especially aquaplaning performance can be improved.